riscv: qemu: add SPL configuration

riscv: qemu: add SPL configuration

Add two new configurations (qemu-riscv{32,64}_spl_defconfig) with SPL
enabled for RISC-V QEMU. QEMU does not require SPL to run U-Boot. The
configurations are meant to help the development of SPL on RISC-V.

The configurations enable RAM as the only SPL boot device. Images must
be loaded at address 0x80200000. In the default boot flow, U-Boot SPL
starts in machine mode, loads the OpenSBI FW_DYNAMIC firmware and U-Boot
proper from the supplied FIT image, and starts OpenSBI. U-Boot proper is
then started in supervisor mode by OpenSBI.

Signed-off-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>